Ethylenediamine-Enabled Sustainable Synthesis of Mesoporous Nanostructured Li2FeIISiO4 Particles from Fe(III) Aqueous Solution for Li-Ion Battery Application

Engineering of nanostructured lithium iron silicate (LFS) particles is pursued via a novel benign synthesis approach seeking to understand the crystalline particle formation process and its impact on energy storage capacity. Specifically, mesoporous Li2FeSiO4 nanostructured particles are synthesized via a novel dual-step process involving organic-assisted hydrothermal precipitation from concentrated Fe(III) (1 mol/L) aqueous solution followed by reductive (5 vol % H2) thermal transformation of the precipitate at 400 °C (LFS400) and 700 °C (LFS700). Scanning and transmission electron microscopy revealed the formation of secondary sub-micron-sized porous agglomerates of unitary primary nanocrystals (∼50 nm for LFS400 and ∼200 nm for LFS700). Both ethylene glycol and ethylenediamine are used as crystallization control additives. It is demonstrated that formation of LFS from Fe(III) precursor is made possible only by the action of ethylenediamine. The obtained LFS particles are found to be predominantly monoclinic as per X-ray diffraction and Rietveld refinement and bear an in situ formed N-doped carbon coating layer as characterized by X-ray photoelectron spectroscopy. TEM coupled with selected area electron diffraction (SAED) analysis confirmed the Rietveld refined XRD phase compositions. The reductive annealing-induced phase transformation sequence leading to LFS crystallization is characterized, and the enabling role of ethylenediamine is discussed. Initial galvanostatic charging–discharging and cyclic voltammetry measurements indicate the annealing temperature of LFS formation to influence the Li-ion storage profile as it shifts from two-phase reaction in LFS700 to solid solution in LFS400—this being attributed to nanostructural changes.